Antibodies to DNA modified by benzo(a)pyrene diol epoxide-1 (BPDE-1) were used to study the influence of chromatin structure on the binding of BPDE-1. The results indicate that the packaging of DNA into its nucleosomal conformation affects the binding of BPDE-1 to DNA. The linker region between core particles is 3-fold more susceptible to modification than the DNA is the core particle conformation. Chromatin regions which are highly enriched in transcribed sequences show an increased susceptibility to carcinogen binding. A gene coding for a 3-methylcholanthrene induced P-450 enzyme has been cloned in pBR322. Various parts of the gene have been subcloned yielding 12 overlapping gene regions.